You don't need 51% of the hashrate to have a reasonable chance of overturning a single confirmation. You can see the relevant calculations in "Section 11. Calculations" of the bitcoin whitepaper.

If someone has 51% of the hashrate (and can sustain this for enough time), then they have a 100% chance to overcome 1 confirmation (or 6, or 20, or 100, provided they can sustain this proportion of the hashrate). However, with only a 1 block deficit, a much lower percent of the hashrate can still give a reasonable chance. 40% of the hashrate has a 82.9% chance to overcome a 1 block deficit, and even only 5% of the hashrate still has a 10.1% chance.

This chance falls exponentially with each additional block. After 6 confirmations, 5% of the hashrate falls from a 10.1% chance to a 0.00038% chance.